1. Field of the Invention
The present invention relates to a lens system that is used in a camera or the like, and more precisely, it relates to a tilting lens system for the focusing or converging of light onto an image plane which is inclined with respect to a principal ray axis (i.e.; an optical axis that is normal to the center of an image plane, such as a photographic film).
2. Description of Related Art
Upon taking a picture with a conventional camera, a swing & tilt operation is effected to focus the image of an object to be taken which is inclined with respect to a plane that is normal to the principal ray axis. The swing & tilt operation is effected when an extension of an object plane, an extension of a plane that is normal to the optical axis of the lens system including a principal point thereof, and an extension of the photographic film plane intersect on a single line (referred to as Scheimpflug's law). Consequently, to effect the swings and tilts in a camera in which the photographic film plane is fixed to the camera body, such as a single lens reflex camera, it is necessary to either adjust the inclination angle of the lens system which is movable or inclinable with respect to the principal ray axis, or to vary the direction of the camera body with respect to the object to be taken to thereby adjust the angle of the principal ray axis with respect to the object. Thus, the center of the object can be made coincidental with the center of the image plane when the requirements of the swing & tilt operation are satisfied in accordance with Scheimpflug's law, as mentioned above. Note that, in general, the terms "swing" and "tilt" mean inclinations of the lens system in the lateral (right and left) direction and in the vertical direction, respectively. The term "tilt" (including tilting or tilted) referred to herein means the inclinations in both the lateral and vertical directions. Namely, in the present invention, the direction of the inclination of the lens system is not limited to a specific direction.
In a known inclinable (tiltable) lens system having first and second principal points which are separated from each other, the inclination of the lens system causes the image of an object to be moved on a photographic film plane. Accordingly, it is necessary to move and adjust the position of the film during the focusing operation in order to take a picture of an object on a predetermined position of the film upon the swing & tilt operation (so-called back focusing). Therefore, it is necessary to make two different adjustments with regard to the center of the object and the focusing of the lens system onto the tilted object. Consequently, these adjustments cause the photographer to be distracted upon taking a picture.
Furthermore, in a conventional camera, the swing & tilt operation can be effected only in a fixed focal length type lens system. Namely, there is no zoom lens which can change the focal length, in which the swing & tilt operation can be effected.